This invention relates to Schottky diodes. More particularly, it relates to Schottky diodes in which minority carrier injection, forward voltage, and metal diffusion are reduced with the use of an intermediary layer including chromium at the metallization-semiconductor interface.
One of the desirable features of a diode is that it should switch rapidly from a conducting state to a non-conducting state when the polarity of the voltage applied to it is switched from a forward biasing condition to a reverse biasing condition. However, in practice, a reverse current usually flows for a short time after the diode is switched to the reverse biased condition. The time taken for this current to become negligible is termed the reverse recovery time, and this time is preferably made as short as possible.
One of the causes of the reverse current which flows during the reverse recovery time is the presence of minority carriers near the diode's junction when the diode is forward biased. When the polarity of the voltage applied to the diode is reversed, these minority carriers flow away from the junction to constitute the reverse current. The number of minority carriers generated determines the total charge that must be removed from the junction area during the reverse current flow, and therefore determines the reverse recovery time. This determines the speed at which diodes can switch. Long reverse recovery times, and hence slow switching speeds, are present in diodes having high levels of minority carrier generation.
Schottky diodes are high-speed diodes which, in theory, minimize the minority carrier generation problem by using only majority carrier conduction. This is accomplished by replacing the semiconductor PN junction in a conventional diode with a metal-semiconductor rectifying junction in which minority carrier generation is significantly lower. Such a metal-semiconductor junction ideally reduces the reverse recovery time of Schottky diodes to zero, and the diodes can therefore switch at high speed.
In practice, minority carrier generation and injection is not completely eliminated in conventional metal-semiconductor Schottky diode junctions. For example, in conventional Schottky diode junctions formed by making contact between an aluminum-based (e.g., Al:1%Si or Al:1.5%Cu:1%Si) metallization layer and an underlying silicon surface, reduced numbers of minority carriers are still generated and injected into the depletion region due to the formation by diffusion of a layer of p-type aluminum doped silicon at the metallization/silicon interface. This can lead to recovery times which are still too long, and can also lead to other effects which can be even more serious.
One such serious effect of minority carrier injection is parasitic transistor operation in an integrated circuit in which Schottky diodes are constructed in an n-type well in a p-type substrate. With this configuration, a significant current can flow to the substrate of the integrated circuit. This current flow can cause the voltage of the substrate to rise and disturb the operation of other parts of the integrated circuit. For example, connection of the substrate contact to ground potential is usually made at a substrate contact pad which may be remote from the Schottky diode. If current flows from the collector of the parasitic PNP transistor to the substrate contact, the resistance of the substrate will cause a voltage drop to appear laterally across a path in the substrate from the Schottky diode to the substrate contact pad. This voltage may be large enough to forward bias junctions of other devices which lie near this path, and thus cause those devices to operate incorrectly. Latch-up in complementary metal-oxide-semiconductor ("CMOS") circuits is an example of such incorrect operation.
The minority carrier injection of a Schottky diode can be further reduced by using a metallization layer of pure aluminum instead of either of the aforementioned AlSi and AlCuSi alloys, but a pure aluminum metallization layer has disadvantageous electrical and metallurgical properties (e.g., low electromigration resistance).
Another problem with some prior Schottky diodes is that the voltage dropped across them when they are forward biased (i.e., the "forward" voltage) may be too high for certain applications, even though this forward voltage is lower than the forward voltage of conventional semiconductor diodes. For a given current, a diode with a lower forward voltage than a conventional diode will have a lower power dissipation than a conventional diode, and this may be critical, for example, in low power applications or where temperature effects are a problem. Additionally, in many cases, Schottky diodes are used as clamps across silicon diodes to prevent conduction of the silicon diode and to eliminate minority carrier storage effects. In these cases, if the forward voltage of the Schottky diode was to be reduced further, there would be a greater voltage margin before the onset of conduction of the silicon diode.
Still another problem with some previously known Schottky diodes, in which a layer of non-aluminum metal (e.g., titanium tungsten) or metal silicide (e.g., platinum silicide) is formed between the aluminum-based metallization layer and the silicon, is the diffusion of aluminum or other metallization material through the metal-semiconductor interface into the silicon. This can degrade the diode's performance significantly because the junction may revert to a aluminum-semiconductor junction rather than one between the intended metal and the semiconductor.
The metal-semiconductor rectifying junction of conventional Schottky diodes have been formed using aluminum evaporation or sputtering techniques. Aluminum is the most commonly used base metal material because Schottky diodes using this material can be fabricated easily and simultaneously with ohmic contacts for other devices in the integrated circuit. As stated above, other materials such as platinum silicide or tungsten titanium, which have desirable properties, also have been used for the metal-semiconductor interface but, unlike aluminum, these typically require that one or more special process steps be added to a conventional integrated circuit process flow, and thus their use in a Schottky diode significantly increases the complexity of the integrated circuit manufacturing process.
In view of the foregoing, it would be desirable to provide a Schottky diode in which minority carrier injection is minimized.
It would further be desirable to provide a Schottky diode in which the forward voltage is reduced.
It would still further be desirable to provide a Schottky diode in which a layer is formed to reduce the diffusion of metal from a metallization layer into the semiconductor.
It would yet further be desirable to provide a Schottky diode wherein the metal-semiconductor interface is formed using a material which is commonly used in conventional integrated circuit manufacture, is relatively easy to process and may be used to form other device structures simultaneously.